Hurlstone, H. the androgen receptor. Our data suggest that acetylation of -catenin in the arm 6 domain name regulates -catenin transcriptional activity by differentially modulating its affinity for Tcf4 and the androgen receptor. Thus, our results describe a new mechanism by which p300 might regulate -catenin transcriptional activity. -Catenin was originally described as a component of cell-cell adhesion complexes, where it binds to E-cadherin. More recently, -catenin was shown to be a key effector of the Wnt signaling pathway, which plays a pivotal role in growth and cell fate F2r at early and late developmental stages (reviewed in recommendations 37, 38, and 49). In the absence of Wnt signals, the (+)-α-Tocopherol cytosolic pool of -catenin is usually maintained at a low level by targeted degradation in a multiprotein complex including the suppressor adenomatous polyposis coli (APC), Axin, glycogen synthase kinase 3, and casein kinase I (16, 30, 41, 52, 53). Wnt activation abrogates the degradation of (+)-α-Tocopherol -catenin and induces its accumulation and translocation into the nucleus, where it binds one of the four members of the T-cell factor/lymphoid enhancer factor (Tcf/Lef) family and activates transcription of target genes (4, 23). Growing evidence has associated Wnt signaling with tumor development. Constitutive Wnt signaling in cancer cells results mainly from genetic defects in the N-terminal region of the -catenin gene itself or in the APC or Axin gene, which induce in all cases the stabilization and nuclear translocation of -catenin (reviewed in reference 38). Although it is well established that the formation of nuclear -catenin/Tcf complexes plays a pivotal role in the activation of Wnt target genes, the fine mechanisms of transcriptional activation and regulation are still under investigation (5, 17). In the absence of -catenin, the Tcf/Lef transcription factors act as transcriptional repressors by recruiting proteins such as (+)-α-Tocopherol Groucho/TLE, CtBP, and histone deacetylase (6-9, 28, 40). Upon Wnt activation, the binding of -catenin to Tcf generates a bipartite transcription factor, in which Tcf provides the DNA binding domain name and the C terminus of -catenin provides the transactivation domain name, therefore inducing a transcriptional switch. Recent physical and biochemical studies of the -catenin-Tcf conversation have provided detailed information on the mode of -catenin recognition by Tcf. Binding regions have been mapped to the N-terminal domain name of Tcf/Lef and armadillo (arm) repeats 3 to 8 of -catenin, with crucial hot spots within repeat 8 (46). The crystal structure of -catenin/Tcf complexes further revealed that the core arm repeat domain of -catenin forms a superhelix of helices, providing a long, positively charged groove that engages the negatively charged -catenin binding domain of Tcf (13, 14, 39). These studies layed out the importance of two crucial lysine residues of -catenin, K312 and K435, called the charged buttons, located in arm repeats 5 and 8. Different aspects of the regulation of Tcf-dependent transcription by -catenin have been unraveled. -Catenin might recruit the basal transcription machinery via its conversation with the TATA-binding protein and Pontin 52 (TIP 49) (+)-α-Tocopherol (3, 18). -Catenin has also been shown to interact with cellular factors essential for its transcriptional activity, such as pygopus and Lgs/BCl9, or with proteins involved in histone modification and chromatin remodeling, such as CBP/p300 and Brahma/Brg-1 (2, 20, 25, 33, 36, 43, 44). A crucial role for CBP/p300 in -catenin/Tcf activity has been exhibited during embryogenesis and -catenin-associated transformation (43, 44). The mechanism by which CBP/p300 stimulate transcription is likely multifactorial (reviewed in recommendations 12 and 27). CBP/p300 can contribute to the formation of a multiprotein activation complex bridging various factors to the general transcription machinery. In addition, CBP/p300 possess intrinsic histone acetyltransferase (HAT) activity, and histone acetylation regulates promoter activity by relieving chromatin-dependent repression. More recently, CBP/p300 have been shown to acetylate a growing number of nonhistone proteins, notably transcription factors such as p53, E2F, HMG I(Y), HNF-4, and human immunodeficiency computer virus Tat (15, 22, 31, 34, 42). Acetylation of these factors may affect different biological functions, including DNA binding affinity, transcriptional activity, stability, and subcellular localization. Recent studies have reported acetylation of -catenin at lysine 49 and acetylation of the Tcf/Lef homolog POP-1 at three neighboring residues (K185, 187, and 188), suggesting that acetylation might also be involved in the transcriptional activity of -catenin-Tcf complexes (11, 50). In this study, we showed that -catenin is usually acetylated in vivo and in vitro by p300. We found that lysine K345, located in arm repeat 6, is an acetyl acceptor and.